Beam-shaping diffusers for ALFOSC to do high-precision photometry

To observe planet transits, high-precision photometry is needed. A standard technique is to defocus the telescope, such that the star image is spread over many pixel. This allows to detect more photons from the object without saturating the detector, increasing the observing efficiency, but more importantly, it limits flat field errors as they are averaged over a larger area, and are less affected by changes in atmospheric seeing. However, the resulting shape of the stellar profile is very irregular, and variable, still effected by atmospheric effects. The use of diffusers can effectively scrambled the incoming light, providing a more constant and optimal (`top-head’) shape, with minimal light loss, significantly reducing any systematic effects in the photometry. Dr. Carolina von Essen from the Stellar Astrophysics Centre, Aarhus University, Denmark, has obtained funding from IDA in Denmark, to purchase such diffusers for use with ALFOSC.

Two Engineered Diffusers^™ were ordered from RPC Photonics.


Name	ID number	Diameter	Thickness	Angle (deg)	FWHM (FASUA)	FWHM (FASUB)	Available	Documentation
Diffuser#1	664	90 mm	3 mm	0.35	4.8"	5.3"	Since Aug 2018	Diffuser#1 sheet and Spectral Coating curve Diffuser#1 throughput and
Diffuser#2	665	90 mm	3 mm	0.50	6.4"	7.5"	Since Oct 2018	Diffuser#2 sheet and Spectral Coating curve Diffuser#2 throughput and Diffuser#2 ghost

The distances from the diffuser filter surfaces to the focal plane are 117.00 mm when mounted in FASUB and 100.50 mm when mounted in FASUA.

To use the diffusers together with a filter, the script "alfosc.wheels" has been updated to take the ID number of the diffuser as an input with the "-d" flag.
For example the items Diffuser#1 and the R-band (#76) filter are entered with:

        alfosc.wheels -d 664 -f 76

The diffusers are open for use to all observers at the NOT. When using one of the new diffusers, please acknowledge Dr. Carolina von Essen from the Stellar Astrophysics Centre, Aarhus University, Denmark. The reference paper for these diffusers is:

von Essen C., Stefansson G., Mallonn M., Pursimo T., Djupvik A. A., Mahadevan S., Kjeldsen H., Freudenthal J., Dreizler S.
First light of engineered diffusers at the Nordic Optical Telescope reveal time variability of the optical eclipse depth of WASP-12b
2019, A&A 628, A115

In 2018 we had a diffuser on loan from Dr. Gudmundur Stefansson (Pennsylvania State University), which has the same characteristics as Diffuser#1, see table above, albeit of a smaller size. With ALFOSC, this diffuser provided a very stable stellar image with a width (~FWHM) of 5.46 arcsec (see for some test results here). Detailed results from data obtained with this diffuser at the ARC 3.5m telescope can be found in Stefansson et al. 2017 ApJ 848. When using the test diffuser on loan during 2018, please acknowledge Dr. Gudmundur Stefansson from the Department of Astronomy & Astrophysics of The Pennsylvania State University, USA, in any publication.

Last modified: April 19 2023